One class of hybrid electric vehicle powertrains is commonly referred to as a power split powertrain that has two sources of power. The first source includes an internal combustion engine and a second source is a combination of an electric motor, a generator and a battery. The engine and the generator, together with a planetary gearset, a countershaft and a motor, establish a mechanical torque flow path and an electromechanical torque flow path to vehicle traction wheels. The battery is an energy storing device for the generator and the motor. Engine power is divided into two power flow paths at any generator speed and vehicle speed. Engine speed is controlled by the generator, which implies that the engine speed can be decoupled from the vehicle speed within the allowed speed range of the generator. This mode of operation is called positive power split when the generator is generating electrical power using mechanical power input from the engine.
Because of the mechanical properties of the planetary gearset, the generator can distribute power to the planetary gearset to drive the vehicle. This mode of operation is called “negative power split”. The combination of a generator, a motor and a planetary gearset thus can be considered to have electrical continuously variable (e-CVT) transmission characteristics.
A generator brake can be activated so that engine output power is transmitted with a fixed gear ratio to the torque output side of the powertrain through a mechanical path only. The first power source can only effect forward propulsion of the vehicle since there is no reverse gear. The engine requires either generator control or application of a generator brake to transmit output power for forward drive.
When the second power source is active, the electric motor draws power from the battery and drives the vehicle independently of the engine for both forward drive and reverse drive. In addition, the generator can draw power from the battery and drive against a one way clutch on the engine power output shaft to propel the vehicle in a forward direction. This mode of operation is called “generator drive mode”. A vehicle system controller coordinates the two power sources so that they work together seamlessly to meet a driver's torque demand without exceeding powertrain system limits. The vehicle system controller allows continuous regulation of engine speed for any given vehicle speed and power request. The mechanical power flow path provides efficient power delivery through the planetary gearset to the driveshaft.
A power split hybrid electric powertrain has disadvantages when it is operated in a negative power split mode. For example, a negative power split establishes a power circulation path within the transmission itself as power is delivered to the traction wheels. This power circulation reduces powertrain efficiency due to the extra power loss generated during power circulation. A negative power split is needed, however, in a power split system in order to regulate engine speed at high vehicle speeds and at a low power demand. This operating condition happens mainly during operation on a highway at high speeds.
A second disadvantage of operating in a negative power split mode is apparent also during reverse drive. A power split powertrain of known design has low torque capacity in reverse drive. Since the planetary gearset cannot provide negative torque output in reverse drive, the motor must be used to balance ring gear torque at the planetary gearset. This limits the reverse drive torque capacity. Further, the generator must be sized large enough so that it is capable of holding engine torque for heavy vehicle applications.